(1) Field of Invention
The present invention relates to an active steering controller for heavy road vehicles with front and rear steerable wheels, such as a bus, or an articulated vehicle such as a tractor-semi-trailer.
(2) Description of Related Art
The use of long and large vehicles, many of which are articulated, is widespread as they have many advantages in terms of their flexibility of operation and large load carrying ability. However, given their length, there can be difficulties in controlling them safely. Their length and size reduce their maneuverability. Attempts have been made to improve the maneuverability of such vehicles by providing additional steerable axles at the rear of the vehicle and by splitting the vehicle into a number of components and articulating them. Such vehicles still have problems, however. In particular, articulated vehicles can be difficult to steer at low speeds, their movement can become cumbersome and it can be difficult to move them through corners and tight curves that smaller, less lengthy vehicles would have no difficulty in navigating. This restricts their use in built-up areas. Furthermore, such vehicles can become difficult to control at high speeds if, for example, emergency evasive action is required by the driver, with a “whip-crack” effect (also known as rearward amplification) occurring in the rear trailer of the vehicle, leading to instability and possible overturning of the vehicle.
Attempts have been made to improve the maneuverability of such vehicles by introducing steering wheels that are normally only steered at low speeds so that the whole vehicle can be driven to follow more closely the path intended by the driver. Such steering arrangements, sometimes called command steer systems, usually involve the provision of steerable wheels at the rear of the vehicle or in the trailer section of the vehicle. These wheels are steered by a steering mechanism or by mechanical actuators controlled by a computer to assist in maneuvering of the vehicle. Such systems can have benefits, but also have problems associated with them. Firstly, it is very difficult for the systems to cope well with both high and low speeds. A system which works well in assisting steering at low speeds to improve the maneuverability of the vehicle can introduce instability at high speeds unless it is disabled. Likewise, a system which operates to improve stability of the vehicle at high speeds can hinder the maneuverability of the vehicle at low speeds. In addition, prior art systems are unable to improve, to a significant degree, the handling characteristics of larger articulated vehicles with multiple trailer axles. Given this limited performance, and given the relative expense of such systems they have not become popular in commercial vehicle operations.